FNAF has no real characters...or plot. It is literally a game where you play as a night-watchman in a restaurant with defective robots. But apparently kids love watching markiplier play it, so they made 3 more games, and then that weird FNAF world game... FNAF was an okay game by itself, but it didn't need sequels, and it was ruined by cancerous fans...
Undertale on the other hand has lots of plot and lots of well developed characters. The game style is simplistic, the story is well written, and it is a more enjoyable game overall. I just hope toby fox doesn't make the same mistake as whoever made FNAF and make 3 pointless games as 'sequels' to undertale. The fanbase becomes more and more cancerous by the day, but at least they aren't making up ENTIRE BACKSTORIES FOR GOSHDARN ROBOTS WHO HAVE NO DARN STORY.
That's my 2 cents.